Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 2011/121043 PCT/EP2011/054951
RECTIFIER BASED TORSIONAL MODE DAMPING SYSTEM AND METHOD
BACKGROUND OF THE INVENTION
[0001] Embodiments of the subject matter disclosed herein generally relate to
methods and
systems and, more particularly, to mechanisms and techniques for dampening a
torsional
vibration that appears in a rotating system.
[0002] The oil and gas industry has a growing demand for driving various
machines at
variable speeds. Such machines may include compressors, electrical motors,
expanders,
gas turbines, pumps, etc. Variable frequency electrical drives increase energy
efficiency
and provide an increased flexibility for the machines. One mechanism for
drivin, foi-
l
example, a large gas compression train is the load commutated inverter (LCI).
A gas
compression train includes, for example, a gas turbine, a motor, and a
compressor. The gas
compression train may include more or less electrical machines and turbo-
machines. A
turbo-machine may be any non-electrical machine. However, a problem introduced
by
power electronics driven systems is the generation of ripple components in the
torque of
the electrical machine due to electrical harmonics. The ripple component of
the torque
may interact with the mechanical system at torsional natural frequencies of
the drive train,
which is undesirable.
[0003] A torsional oscillation or vibration is an oscillatory angular motion
that may appear
in a shaft having various masses attached to it as shown for example in Figure
1. Figure 1
shows a system 10 including a gas turbine 12, a motor 14, a first compressor
16 and a
second compressor 18. The shafts of these machines are either connected to
each other or
a single shaft 20 is shared by these machines. Because of the impellers and
other masses
distributed along shaft 20, a rotation of the shaft 20 may be affected by
torsional
oscillations produced by the rotation with different speeds of the masses
(impellers for
example) attached to the shaft.
[0004] As discussed above, the torsional vibrations are typically introduced
by the power
electronics that drive the electrical motor. Figure 1, for example, shows a
power grid
source (power source) 22 providing electrical power to the LCI 24, which in
turn drives the
shaft 20 of the motor 14. The power and may be an isolated power generator. In
order to
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WO 2011/121043 PCT/EP2011/054951
damp (minimize) the torsional vibrations, as shown in Figure 2, which
corresponds to
Figure 1 of U.S. Patent No. 7,173,399, assigned to the same assignee as this
application,
the entire disclosure of which is incorporated here by reference, an inverter
controller 26
may be provided to an inverter 28 of the LCI 24 and may be configured to
introduce an
inverter delay angle change (AP) for modulating an amount of active power
transferred
from inverter 28 to motor 14. Alternatively, a rectifier controller 30 may be
provided to a
rectifier 32 and may be configured to introduce a rectifier delay angle change
(Au.) for
modulating the amount of active power transferred from the generator 22 to a
DC-link 44
and thus to the motor 14. It is noted that by modulating the amount of active
power
transferred from the generator 22 to the motor 14 it is possible to damp the
torsional
vibrations that appear in the system including motor 14 and compressor 12. In
this regard,
it is noted that shafts of motor 14 and gas turbine 12 are connected to each
other while a
shaft of generator 22 is not connected to either the motor 14 or compressor
12.
[0005] The two controllers 26 and 30 receive as input, signals from sensors 36
and 38,
respectively, and these signals are indicative of the torque experienced by
the motor 14
and/or the generator 22. In other words, the inverter controller 26 processes
the torque
value sensed by sensor 36 for generating the inverter delay angle change (A13)
while the
rectifier controller 30 processes the torque value sensed by the sensor 38 for
generating the
rectifier delay angle change (Au). The inverter controller 26 and the
rectifier controller 30
are independent from each other and these controllers may be implemented
together or
alone in a given system. Figure 2 shows that sensor 36 monitors a part
(section) 40 of the
shaft of the motor 14 and sensor 38 monitors a shaft 42 of the power generator
22. Figure
2 also shows the DC link 44 between the rectifier 32 and the inverter 28.
[0006] However, the rectifier delay angle change (Au) determined by measuring
a torque
of a power generator is not always practical and/or accurate. Accordingly, it
would be
desirable to provide systems and methods that determine the rectifier delay
angle change
(Au) using other approaches.
BRIEF DESCRIPTION OF THE INVENTION
[0007] According to one exemplary embodiment, there is a torsional mode
damping
controller system connected to a converter that drives a drive train including
an electrical
machine and a non-electrical machine. The controller system includes an input
interface
WO 2011/121043 PCT/EP2011/054951
configured to receive measured data related to variables of the converter or
the drive train
and a controller connected to the input interface. The controller is
configured to calculate
at least one dynamic torque component along a section of a shaft of the drive
train based
on the measured data from the input interface, generate control data for a
rectifier of the
converter for damping a torsional oscillation in the shaft of the drive train
based on the at
least one dynamic torque component, and send the control data to the rectifier
for
modulating an active power exchanged between the converter and the electrical
machine.
[0008] According to still another exemplary embodiment, there is a system for
driving an
electrical machine that is part of a drive train. The system includes a
rectifier configured to
receive an alternative current from a power source and to transform the
alternative current
into a direct current; a direct current link connected to the rectifier and
configured to
transmit the direct current; an inverter connected to the direct current link
and configured
to change a received direct current into an alternative current; an input
interface configured
to receive measured data related to variables of the converter or the drive
train; and a
controller connected to the input interface. The controller is configured to
calculate at least
one dynamic torque component of the electrical machine based on the measured
data from
the input interface, generate control data for the rectifier for damping a
torsional oscillation
in a section of a shaft of the mechanical system based on the at least one
dynamic torque
component, and send the control data to the rectifier for modulating an active
power
exchanged between the converter and the electrical machine.
[0009] According to yet another exemplary embodiment, there is a method for
damping a
torsional vibration in a drive train including an electrical machine. The
method includes
receiving measured data related to variables of (i) a converter that drives
the electrical
machine or (ii) the drive train; calculating at least one dynamic torque
component of the
electrical machine based on the measured data; generating control data for a
rectifier of the
converter for damping a torsional oscillation in a section of a shaft of the
drive train based
on the at least one dynamic torque component; and sending the control data to
the rectifier
for modulating an active power exchanged between the converter and the
electrical
machine.
[0010] According to yet another exemplary embodiment, there is a computer
readable
medium including computer executable instructions, where the instructions,
when
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executed, implement a method for damping torsional vibrations. The computer
instructions include the steps recited in the method noted in the previous
paragraph.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated in and constitute a
part of the
specification, illustrate one or more embodiments and, together with the
description,
explain these embodiments. In the drawings:
[0012] Figure 1 is a schematic diagram of a conventional gas turbine connected
to an
electrical machine and two compressors;
[0013] Figure 2 is a schematic diagram of a driving train including rectifier
controller and
inverter controller;
[0014] Figure 3 is a schematic diagram of a gas turbine, motor and load
controlled by a
controller according to an exemplary embodiment;
[0015] Figure 4 is a schematic diagram of a converter and associated logic
according to an
exemplary embodiment;
[0016] Figure 5 is a schematic diagram of a converter and associated logic
according to an
exemplary embodiment;
[0017] Figure 6 is a graph illustrating a torque of a shaft with disabled
damping control;
[0018] Figure 7 is a graph illustrating a torque of a shaft with enabled
damping control
according to an exemplary embodiment;
[0019] Figure 8 is a schematic diagram of a converter and associated logic
according to an
exemplary embodiment;
[0020] Figure 9 is a schematic diagram of a controller configured to control a
converter for
damping torsional vibrations according to an exemplary embodiment;
[0021] Figure 10 is a schematic diagram of a controller that provides
modulation to a
rectifier according to an exemplary embodiment;
[0022] Figure 11 is a flow chart of a method that controls a rectifier for
damping torsional
vibrations according to an exemplary embodiment;
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[0023] Figure 12 is a schematic diagram of a controller that provides
modulation to a
rectifier and an inverter according to an exemplary embodiment;
[0024] Figure 13 is a schematic diagram of voltages existent to an inverter,
rectifier and
DC link of a converter according to an exemplary embodiment;
[0025] Figure 14 is a graph indicating the torsional effect of alpha and beta
angle
modulations according to an exemplary embodiment;
[0026] Figure 15 is a flow chart of a method that controls an inverter and a
rectifier for
damping torsional vibrations according to an exemplary embodiment;
[0027] Figure 16 is a schematic diagram of a voltage source inverter and
associated
controller for damping torsional vibrations according to an exemplary
embodiment; and
[0028] Figure 17 is a schematic diagram of a multimass system.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The following description of the exemplary embodiments refers to the
accompanying
drawings. The same reference numbers in different drawings identify the same
or similar
elements. The following detailed description does not limit the invention.
Instead, the scope
of the invention is defined by the appended claims. The following embodiments
are
discussed, for simplicity, with regard to the terminology and structure of an
electrical motor
driven by a load commutated inverter. However, the embodiments to be discussed
next are
not limited to such a system, but may be applied (with appropriate
adjustments) to other
systems that are driven with other devices, as for example, a voltage source
inverter (VSI).
[0030] Reference throughout the specification to "one embodiment" or "an
embodiment"
means that a particular feature, structure, or characteristic described in
connection with an
embodiment is included in at least one embodiment of the subject matter
disclosed. Thus, the
appearance of the phrases "in one embodiment" or "in an embodiment" in various
places
throughout the specification is not necessarily referring to the same
embodiment. Further, the
particular features, structures or characteristics may be combined in any
suitable manner in
one or more embodiments.
[0031] According to an exemplary embodiment, a torsional mode damping
controller may
be configured to obtain electrical and/or mechanical measurements regarding a
shaft of an
WO 2011/121043 PCT/EP2011/054951
electrical machine (which may be a motor or a generator) and/or a shaft of a
turbo-machine
that is mechanically connected to the electrical machine and to estimate,
based on the
electrical and/or mechanical measurements, dynamic torque components and/or a
torque
vibration at a desired shaft location of a drive train. The dynamic torque
components may
be a torque, a torsional position, torsional speed or a torsional acceleration
of the shaft.
Based on one or more dynamic torque components, a controller may adjust/modify
one or
more parameters of a rectifier that drives the electrical machine to apply a
desired torque
for damping the torque oscillation. As will be discussed next, there are
various data
sources for the controller for determining the damping based on the rectifier
control.
[0032] According to an exemplary embodiment shown in Figure 3, a system 50
includes a
gas turbine 52, a motor 54, and a load 56. Other configurations involving a
gas turbine
and/or plural compressors or other turbo-machines as load 56 are possible.
Still, other
configurations may include one or more expanders, one or more power
generators, or other
machines having a rotating part, e.g., wind turbines, gearboxes. The system
shown in
Figure 3 is exemplary and is simplified for a better understanding of the
novel features.
However, one skilled in the art would appreciate that other systems having
more or less
components may be adapted to include the novel features now discussed.
[0033] The connection of various masses (associated with the rotors and
impellers of the
machines) to a shaft 58 makes the system 50 prone to potential torsional
vibrations. These
torsional vibrations may twist the shaft 58, which may result in significant
lifetime
reduction or even destruction of the shaft system (which may include not only
the shaft or
shafts but also couplings and gearbox depending on the specific situation).
The exemplary
embodiments provide a mechanism for reducing the torsional vibrations.
[0034] To activate the motor 54, electrical power is supplied from the power
grid or a local
generator 60 in case of island or island like power systems. In order to drive
the motor 54
at a variable speed, a load commutated inverter (LCI) 62 is provided between
the grid 60
and the motor 54. As shown in Figure 4, the LCI 62 includes a rectifier 66
connected to a
DC link 68 which is connected to an inverter 70. The rectifier 66, DC link 68,
and inverter
70 are known in the art and their specific structures are not discussed here
further. As
noted above, the novel features may be applied, with appropriate changes, to
VSI systems.
For illustration only, an exemplary VSI is shown and briefly discussed with
regard to
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Figure 16. Figure 4 indicates that the current and voltage received from the
grid 60 are
three phase currents and voltages, respectively. The same is true for the
currents and
voltages through the rectifier, inverter and the motor and this fact is
indicated in Figure 4
by symbol "/3". However, the novel features of the exemplary embodiments are
applicable to systems configured to work with more than three phases, e.g., 6
phase and 12
phase systems.
[0035] LCI 62 also includes current and voltage sensors, denoted by a circled
A and a
circled V in Figure 4. For example, a current sensor 72 is provided in the DC
link 68 to
measure a current iDC. Alternatively, the current in the DC link is calculated
based on
measurements performed in the AC side, for example current sensors 84 or 74 as
these
sensors are less expensive than DC sensors. Another example is a current
sensor 74 that
measures a current iabc provided by the inverter 70 to the motor 54 and a
voltage sensor 76
that measures a voltage vatic provided by the inverter 70 to the motor 54. It
is noted that
these currents and voltages may be provided as input to a controller 78. The
term
"controller" is used herein to encompass any appropriate digital, analog, or
combination
thereof circuitry or processing units for accomplishing the designated control
function.
Returning to Figure 3, it is noted that controller 78 may be part of the LCI
62 or may be a
stand alone controller exchanging signals with the LCI 62. The controller 78
may be a
torsional mode damping controller.
[0036] Figure 4 also shows that an LCI controller 80 may receive mechanical
measurements regarding one or more of the gas turbine 52, the motor 54 and the
load 56
shown in Figure 3. The same may be true for controller 78. In other words,
controller 78
may be configured to receive measurement data from any of the components of
the system
50 shown in Figure 3. For example, Figure 4 shows a measurement data source
79. This
data source may provide mechanical measurements and/or electrical measurements
from
any of the components of the system 50. A particular example that is used for
a better
understanding and not to limit the exemplary embodiments is when data source
79 is
associated with the gas turbine 52. A torsional position, speed, acceleration
or torque of
the gas turbine 52 may be measured by existing sensors. This data may be
provided to
controller 78 as shown in Figure 4. Another example is electrical measurements
taken at
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the converter 62 or motor 54. Data source 79 may provide these measurements to
controller 78 or controller 80 if necessary.
[0037] Controller 80 may generate, based on various references 82, and a
current idx
received from a sensor 84, a rectifier delay angle a for controlling the
rectifier 66.
Regarding the rectifier delay angle a, it is noted that LCIs are designed to
transfer active
power from the grid 60 to the motor 54 or vice versa. Achieving this transfer
with an
optimal power factor involves the rectifier delay angle a and the inverter
delay angle J3.
The rectifier delay angle a may be modulated by applying, for example, a sine
wave
modulation. This modulation may be applied for a limited amount of time. In
one
application, the modulation is applied continuously but the amplitude of the
modulation
varies. For example, as there is no torsional vibration in the shaft, the
amplitude of the
modulation may be zero, i.e., no modulation. In another example, the amplitude
of the
modulation may be proportional with the detected torsional vibration of the
shaft.
[0038] Another controller 86 may be used for generating an inverter delay
angle f3 for the
inverter 70. Modulating the inverter delay angle 0 results in modulating the
inverter DC
voltage which causes a modulation of the DC link current and results in an
active power
oscillation on the load input power. In other words, modulating only the
inverter delay
angle in order to achieve torsional mode damping results in the damping power
coming
mainly from the magnetic energy stored in the DC link 68. Modulation of the
inverter
delay angle results in rotational energy being transformed into magnetic
energy and vice
versa, depending whether the rotating shaft is accelerated or decelerated.
[0039] Further, Figure 4 shows a gate control unit 88 for the rectifier 66 and
a gate control
unit 90 for the inverter 70 that directly control the rectifier and inverter
based on
information received from controllers 80 and 86. An optional sensor 92 may be
located in
close proximity to the shaft of the motor 54 for detecting the dynamic torque
components,
e.g., a torque present in the shaft or a torsional speed of the shaft or a
torsional acceleration
of the shaft or a torsional position of the shaft. Other similar sensors 92
may be placed
between motor 54 and gas turbine 52 or at gas turbine 52. Information ux
regarding
measured dynamic torque components (by sensors 92) may be provided to
controllers 78,
80 and 86. Figure 4 also shows summation blocks 94 and 96 that add a signal
from
controller 78 to signals generated by controllers 80 and 86.
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[0040] According to an exemplary embodiment illustrated in Figure 5, the
torsional mode
damping controller 78 may receive a current lab, and a voltage Vabe measured
at an output
91 of the LCI 62 or the inverter 70. Based on these values (no information
about a
measured torque or speed or acceleration of the shaft of the motor), an air
gap torque for
the motor is calculated and fed into a mechanical model of the system. The
mechanical
model of the system may be represented by several differential equations
representing the
dynamic behavior of the mechanical system and linking the electrical
parameters to the
mechanical parameters of the system. The model representation includes, for
example,
estimated inertia, damping and stiffness values (which can be verified by
field
measurements) and allows to calculate the dynamic behavior of the shaft, e.g.,
torsional
oscillations. The needed accuracy for torsional mode damping may be achieved
as mainly
the accuracy of the phase of the dynamic torque component is relevant for the
torsional
mode damping, and the amplitude information or absolute torque value is less
important.
[0041] In this regard, it is noted that the air gap torque of an electrical
machine is the link
between the electrical and mechanical system of a drive train. All harmonics
and inter-
harmonics in the electrical system are also visible in the air-gap torque.
Inter-harmonics at
a natural frequency of the mechanical system can excite torsional oscillations
and
potentially result into dynamic torque values in the mechanical system above
the rating of
the shaft. Existing torsional mode damping systems may counteract such
torsional
oscillations but these systems need a signal representative of the dynamic
torque of the
motor and this signal is obtained from a sensor that effectively monitors the
shaft of the
motor or shaft components of the motor, such as toothwheels mounted along the
shaft of
the motor. According to exemplary embodiments, no such signal is needed as the
dynamic
torque components are evaluated based on electrical measurements. However, as
will be
discussed later, some exemplary embodiments describe a situation in which
available
mechanical measurements at other components of the system, for example, the
gas turbine,
may be used to determine the dynamic torque components along the mechanical
shaft.
[0042] In other words, an advantage according to an exemplary embodiment is
applying
torsional mode damping without the need of torsional vibration sensing in the
mechanical
system. Thus, torsional mode damping can be applied without having to install
additional
sensing in the electrical or mechanical system as current voltage and/or
current and/or
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speed sensors can be made available at comparably low cost. In this regard, it
is noted that
mechanical sensors for measuring torque are expensive for high power
applications, and
sometimes these sensors cannot be added to the existing systems. Thus, the
existent
torsional mode damping solutions cannot be implemented for such cases as the
existent
torsional mode damping systems require a sensor for measuring a signal
representative of a
mechanical parameter of the system that is indicative of torque. On the
contrary, the
approach of the exemplary embodiment of Figure 5 is reliable, cost effective
and allows
retrofitting an existing system.
[0043] Upon receiving the current and voltage indicated in Figure 5,
controller 78 may
generate appropriate signals (modulations for one or more of Act and A(3) for
controlling
the rectifier delay angle a and/or the inverter delay angle P. Thus, according
to the
embodiment shown in Figure 5, the controller 78 receives measured electrical
information
from an output 91 of the inverter 70 and determines/calculates the various
delay angles,
based, for example, on the damping principle described in Patent No.
7,173,399. In one
application, the delay angles may be limited to a narrow and defined range,
for example, 2
to 3 degrees, not to affect the operation of the inverter and/or converter. In
one
application, the delay angles may be limited to only one direction (either
negative or
positive) to prevent commutation failure by overhead-firing of the thyristors.
As illustrated
in Figure 5, this exemplary embodiment is an open loop as corrections of the
various
angles are not adjusted/verified based on a measured signal (feedback) of the
mechanical
drive train connected to motor 54. Further, simulations performed show a
reduction of the
torsional vibrations when the controller 78 is enabled. Figure 6 shows
oscillations 100 of
the torque of the shaft of the motor 54 versus time when the controller 78 is
disabled and
Figure 7 shows how the same oscillations are reduced/damped when the
controller 78 is
enabled to generated alpha modulation, for example, at time 12 s, while the
mechanical
drive train is operated in variable speed operation and crossing at t = 12s a
critical speed.
Both figures plot a simulated torque on the y axis versus time on the x axis.
[0044] According to another exemplary embodiment illustrated in Figure 8, the
controller
78 may be configured to calculate one or more of the delay angles changes
(modulations)
Au and/or A[3 based on electrical quantities obtained from the DC link 68.
More
specifically, a current irc may be measured at an inductor 104 of the DC link
68 and this
WO 2011/121043 PCT/EP2011/054951
value may be provided to controller 78. In one application, only a single
current
measurement is used for feeding controller 78. Based on the value of the
measured current
and the mechanical model of the system, the controller 78 may generate the
above noted
delay angle changes. According to another exemplary embodiment, the direct
current IDC
may be estimated based on current and/or voltage measurements performed at the
rectifier
66 or inverter 70.
[0045] The delay angle changes calculated by the controller 78 in any of the
embodiments
discussed with regard to Figures 5 and 8 may be modified based on a closed
loop
configuration. The closed loop configuration is illustrated by dashed line 110
in Figure S.
The closed loop indicates that an angular position, speed, acceleration, or
torque of the
shaft of the motor 54 may be determined with an appropriate sensor 112 and
this value
may be provided to the controller 78. The same is true if sensor or sensors
112 are
provided to the gas turbine or other locations along shaft 58 shown in Figure
3.
[0046] The structure of the controller 78 is discussed now with regard to
Figure 9.
According to an exemplary embodiment, the controller 78 may include an input
interface
120 that is connected to one of a processor, analog circuitry, reconfigurable
FPGA card,
etc. 122. Element 122 is configured to receive the electrical parameters from
the LCI 62
and calculate the delay angle changes. Element 122 may be configured to store
a
mechanical model 128 (disclosed in more details with regard to Figure 17) and
to input the
electrical and/or mechanical measurements received at input interface 120 into
the
mechanical model 128 to calculate one or more of the dynamical torque
components of the
motor 54. Based on the one or more dynamical torque components, damping
control
signals are generated in damping control unit 130 and the output signal is
then forwarded
to a summation block and a gate control unit. According to another exemplary
embodiment, the controller 78 may be an analog circuit, a reconfigurable FPGA
card or
other dedicated circuitry for determining the delay angle changes.
[0047] In one exemplary embodiment, controller 78 continuously receives
electrical
measurements from various current and voltage sensors and continuously
calculates
torsional damping signals based on dynamic torque components calculated based
on the
electrical measurements. According to this exemplary embodiment, the
controller does not
determine whether torsional vibrations are present in the shaft but rather
continuously
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calculates the torsional damping signals based on the calculated dynamic
torque value.
However, if there are no torsional vibrations, the torsional damping signals
generated by
the controller and sent to the inverter and/or rectifier are not affecting the
inverter and/or
rectifier, i.e., the angle changes provided by the damping signals are
negligible or zero.
Thus, according to this exemplary embodiment, the signals affect the inverter
and/or
rectifier only when there are torsional vibrations.
[0048] According to an exemplary embodiment, the direct torque or speed
measurement at
the gas turbine shaft (or estimated speed or torque information in the shaft)
enables the
controller to modulate an energy transfer in the LCI in counter-phase to the
torsional
velocity of a torsional oscillation. Damping power exchanged between the
generator and
the LCI drive may be electronically adjusted and may have a frequency
corresponding to a
natural frequency of the shaft system. This damping method is effective for
mechanical
systems with a high Q factor, i.e., rotor shaft system made of steel with high
torsional
stiffness. In addition, this method of applying an oscillating electrical
torque to the shaft of
the motor and having a frequency corresponding to a resonant frequency of the
mechanical
system uses little damping power.
[0049] Therefore, the above discussed controller may be integrated into a
drive system
based on the LCI technology without overloading the drive system. This
facilitates the
implementation of the novel controller to new or existing power systems and
makes it
economically attractive. The controller may be implemented without having to
change the
existing power system, e.g., extending the control system of one of the LCI
drives in the
island network.
[0050] If the LCI operational speed and torque is varied in a large range, the
effectiveness
of the torsional mode damping may depend on the grid-side converter current
control
performance. The torsional mode damping operation results in a small
additional DC link
current ripple at a torsional natural frequency. As a result, there are two
power
components at this frequency: the intended component due to inverter firing
angle control
and an additional component due to the additional current ripple. The phase
and
magnitude of this additional power component is function of system parameters,
current
control settings and point of operation. These components result into a power
component
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that is dependent on current control and a component that is dependent on
angle
modulation.
[0051] According to an exemplary embodiment, two alternative ways of power
modulation
may be implemented by the controller. A first way is to directly use the
current reference
on the grid side (requires fast current control implementation), e.g., a-
modulation with a
damping component. A second way is to modulate the grid-side and the machine-
side
angles, resulting into a constant dc-link current, e.g,, a-f3-modulation with
a damping
frequency component. The current control on the grid-side is part of this
damping control
and therefore, the current control does not counteract the effect of the angle
modulation. In
this way, the damping effect is higher and independent from the current
control settings.
[0052] According to an exemplary embodiment illustrated in Figure 10, the
systems 50
includes similar elements to the system shown in Figures 3 and 4. Controller
78 is
configured to receive electrical measurements (as shown in Figures 4, 5, and
8) and/or
mechanical measurements (see for example Figures 4 and 8 or sensor 112 and
link 110 in
Figure 10) with regard to one or more of the motor 54 or load 56 or the gas
turbine (not
shown) of system 50. Based only on the electrical measurements, or only on the
mechanical measurements, or on a combination of the two, the controller 78
generates
control signals for applying a-modulation to the rectifier 66. In one
application, the a-
modulation is applied to a reference value of the a angle. For example,
current reference
modulation is achieved by a-modulation while the (3 angle is maintained
constant at the
inverter 70. The a-modulation is represented, for example, by Act in both
Figures 4 and 10.
It is noted that this a-modulation is different from the one disclosed in U.S.
Patent no.
7,173,399 for at least two reasons. A first difference is that the mechanical
measurements
(if used) are obtained in the present exemplary embodiment from a location
along shaft 58
(i.e., motor 54, load 56 and/or gas turbine 52) while U.S. Patent no.
7,173,399 uses a
measurement of a power generator 22 (see Figure 2). A second difference is
that according
to an exemplary embodiment, no mechanical measurements are received and used
by the
controller 78 for performing the a-modulation.
[0053] According to an exemplary embodiment illustrated in Figure 11, there is
a method
for damping a torsional vibration in a compression train including an
electrical machine.
The method includes a step 1100 of receiving measured data related to
parameters of (i) a
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WO 2011/121043 PCT/EP2011/054951
converter that drives the electrical machine or (ii) the compression train, a
step 1102 of
calculating at least one dynamic torque component of the electrical machine
based on the
measured data, a step 1104 of generating control data for a rectifier of the
converter for
damping a torsional oscillation in a shaft of the compression train based on
the at least one
dynamic torque component, and a step 1106 of sending the control data to the
rectifier for
modulating an active power exchanged between the converter and the electrical
machine.
[0054] According to another exemplary embodiment illustrated in Figure 12,
system 50
may have both the rectifier 66 and the inverter 70 simultaneously controlled
(i.e., both a-
modulation and (3-modulation) for damping torsional oscillations. As shown in
Figure 12,
controller 78 provides modulations for both the rectifier controller 88 and
the inverter
controller 90. Controller 78 determines the appropriate modulation based on
(i)
mechanical measurements measured by sensor(s) 112 at one of the motor 54, load
56
and/or gas turbine 52, (ii) electrical measurements as shown in Figures 4, 5,
and 8, or both
(i) and (ii).
[0055] More specifically, the a- and (3-modulation may be correlated as
discussed next
with reference to Figure 13. Figure 13 shows representative voltage drops
across rectifier
66, DC link 68 and inverter 70. As a result of the a- and (3-modulation it is
desired that the
DC link current is constant. Associated voltage drops shown in Figure 13 are
given by:
VDCa =k-VACG =cos(a)
vDci = k= VACM cos((3), and
VDCa = vDC(3 + VDCL,
where VACG is the voltage amplitude of the power grid 60 in Figure 12 and VACM
is the
voltage amplitude of the motor 54.
[0056] By differentiating the last relation with time and imposing the
condition that the
change of the VDCL in time is zero, the following mathematical relation is
obtained
between the a-modulation and the (3-modulation:
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WO 2011/121043 PCT/EP2011/054951
d(VDCa)/dt = - k=VAcG'sin(a); and
d(VDCp)/dt = - k=VACM =sin((3);
[0057] which results in:
da = (VAchI'sin((3))/(VACG'sin(a))-d(3.
[0058] Based on this last relation, both the a-modulation and j3-modulation
are performed
simultaneously, as shown, for example, in Figure 14. Figure 14 shows the
actual torque
200 increasing around to = 1.5 seconds. It is noted that no a-modulation 202
or 13-
modulation 204 is applied between to and ti. At ti an excitation 206 is
applied between tl
and t2 and both modulations 202 and 204 are applied. At the end of the time
interval ti to
t2 it is noted that both modulations are removed and the oscillations of the
torque 200 is
exponentially decreasing because of the inherent mechanical damping properties
of the
mechanical drive train. This example is simulated and not measured in a real
system. For
this reason, both modulations are strictly controlled, e.g., are started at ti
and stopped at t2.
However, in a real implementation of the a-modulation and 13-modulation, the
modulations
may be performed continuously with the amplitude of the modulation being
adjusted based
on the severity of the torsional oscillations. An advantage of this combined
modulation
over the (3-modulation is that there is no need for phase adaption at
different operating
points and the LCI control parameters may have no effect on the damping
performance.
This modulation example is provided to illustrate the effect of modulating
both delay
angles on the mechanical system. The simulation result is shown using an open
loop
response to the mechanical system for the torsional damping system with
inverse damping
performance.
[0059] According to an exemplary embodiment illustrated in Figure 15, there is
a method
for damping a torsional vibration in a compression train including an
electrical machine.
The method includes a step 1500 of receiving measured data related to
parameters of (i) a
converter that drives the electrical machine or (ii) the compression train, a
step 1502 of
calculating at least one dynamic torque component of the electrical machine
based on the
WO 2011/121043 PCT/EP2011/054951
measured data, a step 1504 of generating control data for each of an inverter
and a rectifier
of the converter for damping a torsional oscillation in a shaft of the
compression train
based on the at least one dynamic torque component, and a step 1506 of sending
the
control data to the inverter and the rectifier for modulating an active power
exchanged
between the converter and the electrical machine. It is noted that the dynamic
torque
component includes a rotation position, rotational speed, rotational
acceleration or a torque
related to a section of the mechanical shaft. It is also noted that the
expression modulating
an active power expresses the idea of modulation at an instant even if the
mean active
power over a period T is zero. In addition, if a VSI is used instead of an LCI
another
electrical quantity may be modified as appropriate instead of the active
power.
[0060] According to an exemplary embodiment illustrated in Figure 16, a VSI
140
includes a rectifier 142, a DC link 144, and an inverter 146 connected to each
other in this
order. The rectifier 142 receives a grid voltage from a power source 148 and
may include,
for example, a diode bridge or an active front-end based on semiconductor
devices. The dc
voltage provided by the rectifier 142 is filtered and smoothed by capacitor C
in the DC link
144. The filtered dc voltage is then applied to the inverter 146, which may
include self-
commutated semiconductor devices, e.g., Insulated Gate Bipolar Transistors
(IGBT), that
generate an ac voltage to be applied to motor 150. Controllers 152 and 154 may
be
provided for rectifier 142 and inverter 146, in addition to the rectifier and
inverter
controllers or integrated with the rectifier and inverter controllers, to damp
torsional
vibrations on the shaft of the motor 150. The rectifier controller 153 and
inverter
controller 155 are shown connected to some of the semiconductor devices but it
should be
understood that all the semiconductor devices may be connected to the
controllers.
Controllers 152 and 154 may be provided together or alone and they are
configured to
determine dynamic torque components based on electrical measurements as
discussed with
regard to Figures 4 and 5 and influence control references of the build-in
rectifier and
inverter control, e.g., the torque or current-control reference.
[0061] According to an exemplary embodiment illustrated in Figure 17, a
generalized
multimass system 160 may include "n" different masses having corresponding
moments of
inertia J1 to J, For example, the first mass may correspond to a gas turbine,
the second
mass may correspond to a compressor, and so on while the last mass may
correspond to an
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WO 2011/121043 PCT/EP2011/054951
electrical motor. Suppose that the shaft of the electrical motor is not
accessible for
mechanical measurements, e.g., rotational position, speed, acceleration or
torque. Further,
suppose that the shaft of the gas turbine is accessible and one of the above
noted
mechanical parameters may be directly measured at the gas turbine. In this
regard, it is
noted that generally a gas turbine has high accuracy sensors that measure
various
mechanical variables of the shaft for protecting the gas turbine from possible
damages. On
the contrary, a conventional motor does not have these sensors or even if some
sensors are
present, the accuracy of their measurements is poor.
[0062] The differential equation of the whole mechanical system is given by:
J(d02 /dt2) + D (dO/dt) + KO = TeXt
where J (torsional matrix), D (damping matrix), and K (torsional stiffness
matrix) are
matrices connecting the characteristics of the first mass (for example, d10,
d1 , k12, J1) to the
characteristics of the other masses and Text is an external (net) torque
applied to the system,
e.g., by a motor. Based on this model of the mechanical system, a torque or
other dynamic
torque component of the "n" mass may be determined if characteristics of, for
example, the
first mass are known. In other words, the high accuracy sensors provided in
the gas turbine
may be used to measure at least one of a torsional position, speed,
acceleration or torque of
the shaft of the gas turbine. Based on this measured value, a dynamic torque
component of
the motor ("n" mass) or another section of the drive train may be calculated
by a processor
or controller 78 of the system and thus, control data may be generated for the
inverter or
rectifier as already discussed above.
[0063] In other words, according to this exemplary embodiment, the controller
78 needs to
receive mechanical related information from one turbo-machinery that is
connected to the
motor and based on this mechanical related information the controller is able
to control the
converter to generate a torque in the motor to damp the torsional vibration.
The turbo-
machinery may be not only a gas turbine but also a compressor, an expander or
other
known machines. In one application, no electrical measurements are necessary
for
performing the damping. However, the electrical measurements may be combined
with
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WO 2011/121043 PCT/EP2011/054951
mechanical measurements for achieving the damping. In one application, the
machine that
applies the damping (damping machine) is not accessible for mechanical
measurements
and the dynamic torque component of the damping machine is calculated by
mechanical
measurements performed on another machine that is mechanically connected to
the
damping machine.
[0064] The disclosed exemplary embodiments provide a system and a method for
damping
torsional vibrations. It should be understood that this description is not
intended to limit
the invention. On the contrary, the exemplary embodiments are intended to
cover
alternatives, modifications and equivalents, which are included in the spirit
and scope of
the invention as defined by the appended claims. For example, the method may
be applied
to other electric motor driven mechanical systems, such as large water pumps,
pumped
hydro power stations, etc. Further, in the detailed description of the
exemplary
embodiments, numerous specific details are set forth in order to provide a
comprehensive
understanding of the claimed invention. However, one skilled in the art would
understand
that various embodiments may be practiced without such specific details.
[0065] Although the features and elements of the present exemplary embodiments
are
described in the embodiments in particular combinations, each feature or
element can be used
alone without the other features and elements of the embodiments or in various
combinations
with or without other features and elements disclosed herein.
[0066] This written description uses examples of the subject matter disclosed
to enable any
person skilled in the art to practice the same, including making and using any
devices or
systems and performing any incorporated methods. The patentable scope of the
subject
matter is defined by the claims, and may include other examples that occur to
those skilled in
the art. Such other examples are intended to be within the scope of the
claims.
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